That's why the findings of an aviation blogger -- John Butler -- are particularly troubling. They represent a serious compromise in security procedures by allowing passengers to know, via inspecting their barcode, whether they will be subject to conventional screens.

The flaw is specific to the TSA's pre-screening program. That program allows frequent fliers to pay a fee to get to skip certain digital screening requirements. Passengers who pay the fee get to carry on approved liquids in their luggage, don't have to remove their personal electronics, and can keep their belts/shoes on, when travelling through the scanners.

The idea is that the passengers are pre-screened to try to weed out potential violent threats, and then to use the possibility of random screens to deter any would be terrorists from going to great lengths to try to exploit the program.

Except the screens weren't random. According to Mr. Butler, they appear to be pre-determined, and worse yet the barcode on your ticket tells -- without encryption -- whether you will be screened.

The majority of the barcode encodes your name, flight number, departure city, destination city, seat number, etc. But the final encoded number is a mysterious '1' or '3'. The number encodes a number of beeps that prompts the TSA agents -- in predetermined fashion -- whether to screen the passenger (1 beep means no conventional pre-check, 3 beeps means to do a conventional pre-check).

II. Want to Illegally Skip Security? Print a Modified Boarding Pass

As Mr. Butler points out, a malicious flyer could read their bar code information, then re-encode a new bar code with the '3' replaced with a '1'. The blogger summarizes:

What terrorists or really anyone can do is use a website to decode the barcode and get the flight information, put it into a text file, change the 1 to a 3, then use another website to re-encode it into a barcode. Finally, using a commercial photo-editing program or any program that can edit graphics replace the barcode in their boarding pass with the new one they created. Even more scary is that people can do this to change names. So if they have a fake ID they can use this method to make a valid boarding pass that matches their fake ID. The really scary part is this will get past both the TSA document checker, because the scanners the TSA use are just barcode decoders, they don’t check against the real time information. So the TSA document checker will not pick up on the alterations. This means, as long as they sub in 3 they can always use the Pre-Check line.

Sterling Payne, in a comment toThe Washington Post, refused to say whether Mr. Butler's findings were accurate or not. He comments, "TSA does not comment on specifics of the screening process, which contain measures both seen and unseen. TSA Pre Check is only one part of our intelligence-driven, risk-based approach."

According to The Washington Post, many boarding passes come with verification codes, which could prevent the attack from being carried out. However, the publication notes that some boarding passes are marked as "unverified" and appear to still be validated. As boarding passes can be printed up to 24 hours in advance, attackers could have a window of opportunity to analyze and modify an unverified pass.

Chris Soghoian, an advocate at the American Civil Liberties Union, said poor security is nothing terribly new for the TSA. He created a website back in 2006 that allowed people to create fake boarding passes to test TSA security.

He comments on the latest hole, "If you have a team of four people [planning an attack], the day before the operation when you print the boarding passes, whichever guy is going to have the least screening is going to be the one who’ll take potentially problematic items through security. If you know who’s getting screened before you walk into the airport, you can make sure the right guy is carrying the right bags."

The temptation, he points out, might be to use profiling or other tactics, but he notes the ACLU opposes them. He says such methods are unnecessary, if the TSA just did its job and encrypted the information on the passes. At the end of the day that's the same conclusion Mr. Butler came to.

Both men made it clear that they did not test the attack by printing fake boarding passes. Mr. Butler stated that he believed that was a "legally grey area and morally black one", while The Washington Post suggests, "[It] is illegal to tamper with a boarding card under U.S. law."

quote: You don't even have to have a real boarding pass. Download a copy of photoshop and build your own.

Nonsense, there is no need to go to such trouble as using photoshop.

Most of the boarding passes are put out as a PDF file. With acrobat, you can save a copy and just change dates and numbers of the original and reprint. Know someone who did this regularly did this to change his boarding status so he could get overhead space and never had a issue.

Same thing can be done to change the dates on a boarding pass or other numbers, just paste a new number over the old and as long as the font matches, no problem. People already do this.